Effect of Primary Particle Size Distribution on Aggregate Stability at Different Size Classes

Particle size distribution (PSD) is one of the most important soil physical properties consisting of the primary and secondary PSD. The purpose of this study was to investigate the relationship between the primary PSD (sand, silt and clay) and  the secondary PSD at different scales of aggregates. Sampling was done from two soil layers of 0-10 and 10-20 cm at farms with different cultivation systems including wheat, barley, maize, alfalfa, fallow and tilled lands, with three replications. Each soil sample was divided into three subsamples (size classes) of 0-0.25mm, 0.25-1mm and 1-5 mm. Results showed that the cultivation system had significant effects (p<0.01) on the secondary PSD. There was a negative significant correlation between the sand percentage and water stable aggregates(WSA) >0.25mm, while the correlation was positive for the silt fraction. Moreover, clay content had no significant influence on  the WSA>0.25mm. The percentages of sand, silt and clay particles in all the  aggregate classes were significantly different, so that with increasing the aggregates size, the sand fraction increased, while the silt and clay contents significantly decreased. The fine, medium and large aggregate classes consisted of 45.5, 55.9 and 60.8 percent of sand, 36.0, 27.5 and 24.3 percent of silt and finally 17.8, 16.6 and 14.8 percent of clay, respectively. However, in the all classes, the content of sand was higher than those of  silt and clay. In addition, with increasing aggregate size from 0.25-1 mm to 1-5 mm, the percentage of WSA>0.25mm increased, significantly. The results also indicated that in these two classes, the sand fraction had negative relationship with the WSA>0.25mm, while the relationships for silt and clay particles were positive. The findings of this research revealed that although the content of sand was higher in larger aggregates, but sand particles led to aggregate instability, however silt and particularly clay particles improved aggregate stability.